This invention relates to microwave ovens and to sub-assemblies for fitting to oven cavities to form microwave ovens.
Reducing the sensitivity of microwave oven heating and cooking performance to load volume, load placement and load properties has been a longstanding challenge for microwave and microwave-convection oven designers.
One of the main reasons for this problem is the relatively direct coupling used between the magnetron source, a free-running oscillator, and the multimode oven cavity with its discrete spatial energy patterns. Each of these energy patterns can only be excited in an oven cavity at its distinct natural (or mode) frequency. That natural frequency is determined by the cavity geometry and dimensions and is modified dynamically by varying load (food and utensil) properties such as size, shape, composition and location within the oven.
To deliver power to the cavity, most microwave ovens employ a fixed feed or excitation system, (e.g., a waveguide) which results in effective power delivery to only those natural energy patterns which have their frequency and field orientation matched to that of the exciting field and which have their maximum field strength location near the feed structure. At the same time, the actual oscillator source (magnetron) frequency and power level is determined by the load impedance it sees. Significant downward or upward shifts in the oven""s natural energy pattern frequencies occur when loading the oven with any dielectric or conducting material.
The invention aims to tackle these problems. The invention has been devised following theoretical and experimental work, as a result of which it has been found that certain components should have particular dimensions, or ranges of dimensions, to suit parameters such as the normal operating frequency of the magnetron which generates the microwave power. In the following description and claims all dimensions are given on the understanding that the tolerance is plus or minus 5%.
According to one aspect of the invention a launch box sub-assembly for fitting to the wall of an oven cavity comprises a metal launch box an open side of which is attached to a metal choke plate having a central launch or coupling aperture for communication with the oven cavity through a hole in the wall thereof, the choke plate being formed so as to prevent or substantially prevent leakage of microwave energy between the launch box sub-assembly and the wall of the oven cavity.
The launch box sub-assembly, together with a metal match plate and means for mounting the match plate within the oven cavity, preferably constitute a set of parts enabling an oven cavity, provided with the hole in one wall, to be made into a microwave oven, optionally with a forced hot air system for subjecting food in the oven cavity to heating by exposure to a forced flow of hot air in addition to exposure to microwave energy.
According to another aspect of the invention there is provided a pair of sub-assemblies for attachment to a wall of an oven cavity in order to form a microwave oven, the first sub-assembly comprising a metal launch box and choke plate for attachment to one side of the wall and the second sub-assembly comprising a metal match plate and mounting means for attaching the match plate on the other side of the wall so that the match plate cooperates with the wall to define a gap for delivering microwave energy to the oven cavity, the choke plate being shaped to prevent or substantially prevent leakage of microwave energy between the wall and the launch box and choke member.
According to a further aspect of the invention there is provided a microwave oven having an oven cavity with a wall formed with a hole covered on an external side of the wall by a metal launch box and choke plate, on an internal side of the wall and within the oven cavity there being fitted a metal match plate which faces the launch box through the hole so that the launch box and match plate form a launch cavity for delivering microwave energy to the oven cavity, the choke plate being shaped such that the periphery of the hole presents low impedance to prevent or substantially prevent leakage of microwave energy between the wall and the launch box and choke plate.
The choke plate preferably has an annular ridge or rib which defines a choke channel and the crest of which is formed by an annular wall which closes the end of the choke channel so as to ensure that this annular area is a zero power or low impedance area. Similar zero power conditions will apply at a distance from the annular wall of one half wavelength of the microwave energy. For a magnetron with an operating frequency of 2450 megahertz, this wavelength is 122 mm in free space, so by making the distance from the periphery of the hole to the center of the annular ridge or rib a distance of 61 mm, a zero power condition will apply around the complete periphery of the circular hole in the wall, a condition which will prevent the leakage of microwave energy between the wall and the launch box. This enables the launch box and choke plate to be attached to the wall by threaded studs and nuts or nuts and bolts, in a simple clamping action, in contrast to prior arrangements where welding has been necessary.
A waveguide is preferably attached to the launch box through a restricted aperture iris which decouples the magnetron from the load presented thereto by the launch box. The launch box preferably accommodates a rotatably driven frequency stirrer that operates as a phase modulator to change the phase angle of the load plane of the magnetron.
Thus, by recourse the invention, manufacturers can produce oven cavities with circular holes in any chosen wall (top, side or back), the circular hole being of known diameter. A circular shape of hole is preferred but in principle the hole could be of any chosen shape, for example polygonal or D-shaped. In each case, the choke channel surrounds the hole so that there is a distance of one half wavelength between the edge of the hole and the center of the choke channel. The launch box and waveguide can be separately welded to the choke plate to form a first sub-assembly. The match plate can be separately attached to the mounting means to form a second sub-assembly, and the two sub-assemblies can then be clamped in position on respective sides of the cavity wall, the sub-assemblies overlapping the hole.
According to a yet further aspect of the invention there is provided a microwave oven having an oven cavity with a wall formed with a hole covered on an external side of the wall by a metal launch box and choke plate, on an internal side of the wall and within the oven cavity there being fitted a metal match plate which faces the launch box through the hole so that the launch box and match plate form a launch cavity for delivering microwave energy to the oven cavity, the launch box having a width of 238 mm, a depth of 238 mm and a height of 56 mm, the match plate being mounted in a plane spaced from the plane of the wall by a distance of 20 mm, so that the launch cavity has a depth of 76 mm, which is equal to half the guide wavelength of the launch box.
In an additional aspect the invention provides a pair of sub-assemblies for attachment to a wall of an oven cavity in order to form a microwave oven, the first sub-assembly comprising a metal launch box and choke plate for attachment to one side of the wall and the second sub-assembly comprising a metal match plate and mounting means for attaching the match plate on the other side of the wall so that the match plate cooperates with the launch box to define a launch cavity for delivering microwave energy to the oven cavity, the launch box having a width of 238 mm, a depth of 238 mm and a height of 56 mm, the mounting means mounting the match plate in a plane spaced from the plane of the wall by a distance of 20 mm, so that the launch cavity has a depth of 76 mm. This arrangement ensures that the match plate and the facing panel of the launch box maintain their desired parallel relationship and desired spacing, even for a hot oven and after repeated heating and cooling.
The choke plate preferably has a rectangular launch or coupling aperture formed therein, preferably having a width of 210 mm and a depth of 200 mm, the match plate being mounted symmetrically below the launch or coupling aperture.
The match plate is preferably positioned and dimensioned to provide efficient microwave coupling to the oven cavity in a way which is independent of the extent of the food load in the cavity and the position of the food load in the cavity. The match plate acts as an over-sized non-contacting cover plate to provide cyclically time-varying edge coupling of energy via fringing and leaky-wave fields from the launch box to the oven cavity field patterns, through the gap defined between the edges of the match plate and the oven wall.
Thus, the first assembly acts as a transducer-exciter which isolates the magnetron from the oven cavity impedance, modulating the phase angle of the effective load plane of the magnetron and sets the preferred SWR (standing wave ratio) ranges which the magnetron tube will see. It also excites amplitude and phase modulated travelling waves along the perimeter of the match plate in a manner which significantly increases the opportunity for energy transfer from the launch box to the time and frequency varying energy patterns in the oven cavity.
One of the main practical advantages of the invention is that all kinds of metal utensils can be used in the oven cavity without sparking or arcing and without the food load being affected. It also allows for multiple shelf cooking with the same or different foods placed on any shelf, with all foods unaffected by the proximity or presence of any other food in the cavity.
The match plate is preferably rectangular, preferred dimensions being 272 mm wide by 342 mm deep for an oven cavity with a width of 609 mm, a depth of 457 mm and a height of 441 mm.